Method of producing aluminium



Dec. 27, 1966 J. SCHMITT 3,294,656

METHOD OF PRODUCING ALUMINIUM Filed Oct. 16, 1962 2 Sheets-Sheet 1ATTORN y5,

Dec. 27, 1966 J. SCHMITT 3,294,656

METHOD OF PRODUCING ALUMINIUM Filed Oct. 16, 1962 2 Sheets-Sheet 2 l l fl l 1 I ATT NEYS.

BY W fw? M United States Patent 3,294,656 METHOD 0F PRGDUCIN G ALUMINIUMJohannes Schmitt, Rheinfelden, Baden, Germany, assignor to SwissAluminium Ltd., Chippis, Switzerland, a jointstock company ofSwitzerland Filed Oct. 16, 1962, Ser. No. 230,855 Claims priority,application Switzerland, Oct. 17, 1961, 12,030/ 61 4 Claims. (Cl.204-67) The present invention relates to a process of producingaluminium in an electrolytic furnace from a fused iluoride (cryolite orchiolite) electrolyte containing alumina.

Processes for operating furnaces having one or more pre-baked orself-baking carbon anodes are well-known. `In accordance with thepresent invention, a set of conditions under which aluminium can beproduced at a particularly high current eiciency over long periods oftime has been established, I have found that the temperature of theelectrolyte should be maintained between 940 and 960 C., theconcentration of alumina in the electrolyte should be maintained between5 and 7% by weight and there should be an excess of between 5 and 7% byweight of aluminium fluoride over that combined in the cryolite as3NaF.AlF3.

The current eiciency at any moment can be calculated by anewly-developed and rapid method based on the concentration of carbondioxide and carbon monoxide in the gases escaping from the anodes (theanode gases) at that moment according to the formula:

Momentary -current efciency (percent):

Samples of anode gases were taken through vertical holes in the anodes.The current efficiency over a long period of operation was determinedstatistically from a number of momentary values.

By the aforesaid method for determining the current efciency, theinuence of the distance of the lower surface of the anode from theseparated aluminium (referred to hereinafter as the electrode distance),the electrolyte temperature, the concentration of alumina in theelectrolyte and its acidity (the excess of aluminium fluoride over thatamount which is combined in the cryolite as 3NaF.AlF3), on the currentefciency, as Well as its course between two anode effects has beendetermined. The influence of one of these variables on the currentefliciency while the remaining variables were maintained substantiallyconstant was tested. FIGURES 1 to 4 of the accompanying Edrawingsillustrate the results obtained. All precentages are percentages byweight.

FIGURE 1 shows the inuence of the electrode distance (the distance ofthe lower surface of the anode from the separated aluminium) and theassociated furnace Voltage (voltage drop between the anode and cathodecurrent leads outside the electrolytic furnace) on the currentefiiciency. The measurements were carried out at a temperature of 970 C,with a cryolite electrolyte which contained 'an excess of AlF3 of about3% and a concentration -of alumina of about 4.7%. From the curve shown,which was obtained from 143 separate measurements, it surprisinglyappears that the electrode distance has a practically immaterialinfluence on the current ethciency. 0n increasing the electrode distancefrom 4 to 6 cm., the current eiciency remained practically constant.With a greater distance, it increased slightly.

FIGURE 2 shows the influence of the electrolyte ternperature on thecurrent efficiency. The experiments were carried out with a cryoliteelectrolyte which contained an excess of A1F3 of about 1.5% and an A1203content of about 6.7%. The curve, which was obtained from 94 individualmeasurements, shows that the current efficiency decreased rapidly withincreasing temperature to a surprising extent. At 950 C., the currentefficiency was found to be about 86.5%, while at 1000 C. it was only80.5%.

FIGURE 3 shows the inuence of the concentration of alumina in theelectrolyte on the current efciency. The measurements were carried outin a cryolite electrolyte at 970 C. with an excess of AlF3 of about3.3%. From the curve, which was obtained from separate measurements, itcan be seen that the current efliciency increased rapidly withincreasing A1203 concentration.

FIGURE 4 shows the iniuence of the A1F3 excess in the cryoliteelectrolyte on the current eiiiciency. The measurements were carried outin an electrolyte at 965 C. which had ian A1202 concentration of about5.2%. From the curve, which was obtained from 176 separate measurements,it can be seen that the current eiciency first decreased sharply withIan increasing excess of AlF3 up to about 4%, and then increasedsharplyagain from about 5%.

I thus found that the current efficiency was influenced to a surprisingextent by the electrolyte temperature, the concentration of alumina andthe excess of aluminium uoride in the electrolyte, while the electrodedistance in contrast did not strongly influence the current eiciency.

The curves shown in FIGURES 1 to 4 are characteristic in their shape fora wide variety of electrolytic furnaces. The absolute value of thecurrent eciency, however, depends upon each individual type of furnace.

It was from these results that the process according to the inventionwas developed. In contrast to it, conventional processes are usuallyoperated with an electrolyte temperature of from over 960 C. up to about1000" C., with a concentration of alumina from about 2 to 5% and with anexcess of aluminium fluoride from about 0 to 4%.

If the temperature of the electrolyte falls below 940 C., too strong acrust tends to form on the furnace pot, since the solidificationtemperature of the electrolyte, depending upon its composition (amountsto CaF2, MgF2, as well as impurities such as P205), is generally from910 to 930 C. Too high a temperature (higher than 960 C.) reduces thecurrent efficiency and must therefore be avoided.

Only within a narrow range of alumina concentrations were comparativelyhigh current eiiiciency values obtained. If the A1203 concentration is.allowed to sink to below 5%, the current eiciency falls substantially.If so much alumina is introduced into the electrolyte when the crust isbroken that its alumina-content rises above 7%, theralumina falls to thebottom of the pot and forms a sludge.

The excess of aluminium fluoride should be maintained constant as far aspossible between 5 and 7% by frequent adjustment of the electrolytecomposition, so that the minimum current eiciency at a 4% excess of AlF3is avoided. Although the eiciency increases again when the excess ofaluminium iluoride falls below 4%, working under these conditions isundesirable, since the walls of the pot may become worn because thealumina becomes too soluble in the electrotyte, and in consequence i-tmay not be possible to produce aluminium having a low silicon content.0n the other hand, an aluminium iluoride excess of above 7% isundesirable because the solubility of A1203 becomes greatly reduced andthe furnace may become more difficult to operate.

1n order to keep within the conditions of the process according to theinvention, it has been found to be advantageous to break the crustpresent on the electrolyte at intervals of 1A to 3 hours, preferablyevery 1 to 2 hours. Breaking of the furnace crust at such shortintervals of time is preferably effected by mechanicalmeans,

preferably automatically. The so-called furnace service includes inparticular the breaking of the crust and the supply of alumina as Wellas aluminium fluoride. To -avoid supersaturation of the electrolyte withA1203, it may be advantageous to break only parts of the crust insteadof the whole crust.

Attempts have been made in the past to eliminate the anode effect, inorder to avoid the increased voltage which then arises. I haverecognized that the anode effect cannot be eliminated, and that toimprove the current efficiency even more, it is necessary to cause theanode effect to occur at least once every 48 hours and at the most twiceevery 24 hours. The anode effect influences the purification of theelectrolyte, in that during the anode effect, both gas-formingimpurities and also slag and oxides are driven out of the electrolyte.Moreover it brings about a uniform A1203 concentration.

I have also examined the dependence of the anode effect upon the A1203concentration and on the temperature. Twenty electrolytic furnaces weredeprived of current for about half an hour and in this way theelectrolyte temperature was reduced by 25 C. from 965 C. to 940 C. Afterswitching it on again, samples of electrolyte were taken and temperaturemeasurements were begun when the current intensity reached 40,000 amps.At half-hour intervals, specimens of electrolyte were with drawn atpreviously prepare-d positions for determination of thealumina-concentration and the corresponding electrolyte temperature wasmeasured. The aluminacontent d-uring the anode effect was based lon thatin the last specimen of electrolyte obtained before the anode effect.

FIGURE 5 shows the dependence of the A1203 concentration during theanode effect on the electrolyte ternperature (the curve was obtainedfrom 51 individual measurements). The occurrence of the anode effect isfavored by a reduction in the temperature in that it begins at -a higherA1203 content. For example, at an electrolyte temperature of 995 C., theanode effect occurred only at an A1203 concentration of about 0.6%,while it took place at 960 C. with an A1203 concentration of as much as2.5%. It was found that the furnace lmust be operated as cold aspossible in order that the anode effect should take place with arelatively high content of alumina, which is necessary to achieve asatisfactory current efficiency. l prefer not to wait simply for anodeeffects, for the electrolyte temperature may increase too much. In orderto let the electrolytic furnace work at a temperature between 940 C. and960 C., therefore, I prefer to break the crust periodically and sothrottle the addition o-f A1203 in order to produce an anode effect.lFor that purpose, I allow the concentration of A1203 to drop to -alevel between 3 and 2.5% at least once every 48 ho-urs and at the mosttwice every 24 hours. Also alumina is preferably added mechanically atintervals of 1A to 3 hours.

By the process according to the invention, it is possible to `achieve acurrent efficiency of to 96%.

What is claimed is:

1. The method of operating -an electrolytic furnace either with prebakedanodes or with selfbaking Sderberg carbon anodes for the production ofaluminum by passing direct electric current through a fluorideelectrolyte composed mainly of molten cryolite 3NaF.AlF3 and containingan excess of aluminum fluoride AlF3 above that amount which is combinedin the cryolite as Well as dissolved alumina Al203, comprisingmaintaining during a period of at least 12 hours and at most 48 hoursthe electrolyte temperature between 940 and 960 C., the concentration ofalumina between 5 and 7% by weight and of aluminum fluoride excessbetween 5 and 7% by weight, and preventing the concentration of aluminaand of excess aluminum fluoride from dropping below 5% during saidperiod by frequent additions of alumina and aluminum fluoride to theelectrolyte.

2. The method according to claim 1, in which at least every 48 .hoursand at most every 12 hours, the concentration of alumina is allowed tofall to a level of 3 to 2.5% by weight until an-odic effect occurs.

3. The method according to claim 2, wherein the concentration .ofalumina and excess aluminum fluoride is prevented from droppin-g lbelow5% by weight during said period by frequently breaking the -furnacecrust formed on the electrolyte during saidperiod and adding alumina andaluminum fluoride to the electrolyte during said period sufficiently tomaintain the concentration of the alumina in the electrolyte between 5and 7% by weight during said period and the concentration of excess`aluminum fluoride in the electrolyte between 5 and 7% by Weight duringsaid period.

4. 'I'.he method according to claim 2, wherein the concentration ofalumina and excess alumin-um fluoride is prevented from dropping below5% by Weight during said period by frequently breaking the furnace crustformed on the electrolyte during said period at intervals of 1A to 3hours, and adding alumina and aluminum fluoride to the electrolyteduring said period sufficiently to maintain the concentration of thealumina in the electrolyte between 5 and 7% by Weight during said periodand the concentration of excess aluminum fluoride in the electrolytebetween 5 and 7% by weight during said period.

References Cited bythe Examiner UNITED STATES PATENTS 2,933,440 4/1960Greenfield 240-'67 3,029,194 4/ 1962 De Varda 204-67 3,034,972 5/ 1962Lewis 204-67 JOHN H. MACK, Primary Examiner.

H. S. WILLIAMS, `Assistant Examiner.

1. THE METHOD OF OPERATING AN ELECTROLYTIC FURNACE EITHER WITH PREBAKEDANODES OR WITH SELFBAKING SODERBERG CARBON ANODES FOR THE PRODUCTION OFALUMINUM BY PASSING DIRECT ELECTRIC CURRENT THROUGH A FLUORIDEELECTROYTE COMPOSED MAINLY OF MOLTEN CRYOLITE 3NAF.ALF3 AND CONTAININGAN EXCESS OF ALUMINUM FLUORIDE ALF3 ABOVE THAT AMOUNT WHICH IS COMBINEDIN THE CRYOLITE AS WELL AS DISSOLVED ALUMINA AL2O3, COMPRISINGMAINTAINING DURING A PERIOD OF AT LEAST 12 HOURS AND AT MOST 48 HOURSTHE ELECTROLYTE TEMPERATURE BETWEEN 940* AND 960*C., THE CONCENTRATIONOF ALUMINA BETWEEN 5 AND 7% BY WEIGHT AND OF ALUMINUM FLUORIDE EXCESSBETWEEN 5 AND 7% BY WEIGHT, AND PREVENTING THE CONCENTRATION OF ALUMINAAND OF EXCESS ALUMINUM FLUORIDE FROM DROPPING BELOW 5% DURING SAIDPERIOD BY FREQUENT ADDITIONS OF ALUMINA AND ALUMINUM FLUORIDE TO THEELECTROLYTE.